1. Field of the Invention
The invention relates generally to the suppression of interference amongst communication signals. More particularly, the invention relates to the implementation of statistical techniques to interference suppression.
2. Description of the Related Art
Radios may receive three forms of signals: noise, interference and communication. Noise, created in the atmosphere due to natural events such as lightning and the like, is largely unpredictable. Few steps are taken towards its elimination. Interference signals can be caused inadvertently, such as by several stations broadcasting on the same band, or can be caused deliberately, such as by an adversary transmitting signals to mask communication signals. A variety of techniques have been devised to combat interfering signals.
As major sources of radio interference are non-Gaussian in structure, a technique known to be effective in detecting communication signals in the presence of non-Gaussian interference has employed algorithms that estimate the statistics of the interference. This estimate has been used to transform received radios signals into perceptible communication signals.
One such estimation technique creates a probability density function (PDF) from discrete data. A transform is then created based upon the derivative of the natural log of the PDF corresponding to a magnitude, such as signal amplitude. Researchers have estimated PDFs of interfering signal amplitudes by creating histograms of received data. Finite difference operations were then used to estimate required derivatives.
The histogram technique has inherent inefficiencies that prevent its use with effective two-dimensional adaptive algorithms. This approach is known to be numerically unstable and, to be satisfactory, requires that a relatively great number of signal samples be taken. This high sample requirement makes the histogram method computationally demanding and confines its use mainly to dedicated microprocessors, and then primarily for those used with radios that operate in the lower portion of the radio frequency spectrum (low frequency [LF], very low frequency [VLF], extra low frequency [ELF]). For higher frequency ranges, sufficient sampling becomes extremely difficult.
In environments in which interference characteristics are radically and rapidly changing, the necessity to collect a large number of samples over time makes alternative approaches to the histogram technique worth pursuing.
In one such alternative, joint PDFs are estimated as a product of two one-dimensional PDFs but this approach could lead to a loss of performance for some types of interference.
There is thus a need for an interference suppression system that can be used with dedicated or non-dedicated microprocessors, that is suitable for use with radio receivers operating over both lower and upper portions of the radio frequency spectrum and that can provide interference suppression in radically and rapidly changing interference environments without the loss of system performance.